Electric vehicles have been increasingly advanced, and the number of electric motors installed in vehicles has also been increasing year by year. In line with this trend, a control function needed for each motor is also becoming more sophisticated. The use of brushless motors for vehicles has thus been rapidly expanding.
To control smooth rotation of motor, a brushless motor needs to identify the position of rotor. For this purpose, a conventional motor has a main field magnet and a sensor magnet for detecting rotor position on a motor rotation shaft, and a magnetic detection element is disposed on a circuit board as a magnetic detection means. This is a known general structure for detecting the rotor position.
FIG. 4, FIG. 5, and FIG. 6 are sectional views illustrating examples of a structure of the conventional brushless motor having the above structure for detecting the rotor position.
The conventional brushless motor in FIG. 4 includes magnetic circuit unit 80 including stator 84 and rotor 85, electronic circuit unit 90 including an electronic circuit and a component housing the electronic circuit, and partition 73 for separating spaces of magnetic circuit unit 80 and electronic circuit unit 90. This brushless motor has a structure for detecting the rotor position such that sensor magnet 86 for detecting the position of rotor 85 is disposed facing magnetic detection element 95, which is a means for detecting magnetism, disposed on circuit board 92 in electronic circuit unit 90 with partition 73 in between.
In the conventional brushless motor in FIG. 5, through hole 73h for passing motor rotation shaft 85 through partition 73 is created, and sensor magnet 85 for detecting the position of rotor 85 is held at a tip of rotation shaft 85a passing through this through hole 73h. The brushless motor has a structure for detecting the rotor position such that sensor magnet 86 is disposed facing magnetic detection element 95, which is a means for detecting magnetism, disposed on circuit board 92 in electronic circuit unit 90.
In a brushless motor in FIG. 6 disclosed in PTL1, magnetic circuit unit 80 and electronic circuit unit 90 exist in the same space, and the partition is eliminated. This brushless motor has a structure for detecting the rotor position such that one end face of main field magnet 85b of rotor 85 faces magnetic detection element 95, which is a means for detecting magnetism, disposed on circuit board 92.
In a motor used under the high-temperature environment such as for vehicles, it is essential to protect electronic components mounted on the circuit board, including the magnetic detection element, from an influence of heat. In particular, it is important to place a motor used near an engine compartment of vehicle (environment whose surrounding temperature is 90° C. or above) in a position that the electronic components are less exposed to heat generated from the motor magnetic circuit unit, such as a coil. In other words, in the conventional brushless motor as configured above, the electronic components, including magnetic detection element 95, mounted on circuit board 92 need to be protected from an influence of heat generated by magnetic circuit unit 80.
Still more, magnetic detection element 95 disposed on circuit board 92 requires a relative positional accuracy to a rotor magnetic pole, in order to detect rotor magnetism generated by sensor magnet 86 and output a voltage corresponding to a strength of magnetism.
To further protect the electronic components from the heat generated by magnetic circuit unit 80 under the above high-temperature environment, the partition is thickened to prevent heat transfer to electronic circuit unit 90 in the conventional brushless structure in FIG. 4. However, this has a risk that a magnetic flux generated by sensor magnet 86 may not reach magnetic detection element 95, failing to interlink. A strong magnetic force is thus needed, resulting in increasing the cost.
In the structure of the conventional brushless motor in FIG. 5, sensor magnet 86 and rotor 85 cannot be assembled simultaneously in an assembly process. Therefore, a separate magnetizing process needs to be provided for magnetizing main field magnet 85b and sensor magnet 86 of rotor 85. This may degrade the accuracy for detecting the rotor position. Furthermore, through hole 73h created in partition 73 makes heat enter electronic circuit unit 90, and thus the electronic components, including magnetic detection element 95, mounted on circuit board 92 are affected by the heat. Reliability of electronic circuit unit 90 may thus degrade. Creation of the through hole in circuit board 92 also reduces a mounting area of electronic components and flexibility in circuit pattern design. This may enlarge the size of equipment.
In the structure of the brushless motor of PTL1 shown in FIG. 6, no partition for heat insulation is provided. An influence of heat from magnetic circuit unit 80 directly affects the electronic components, and thus reliability of electronic circuit unit 90 may degrade. In particular, magnetic detection element 95, such as a hall element, largely changes its characteristic by heat. Magnetic detection element 95 itself may be damaged by heat and cause erroneous operation due to functional failure, in addition to a concern of degraded positional accuracy.